synthesisandcharacterizationofphotocatalytic 2-znfe2o4...

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2006, Article ID 45712, Pages 1–4DOI 10.1155/JNM/2006/45712

Synthesis and Characterization of PhotocatalyticTiO2-ZnFe2O4 Nanoparticles

Sesha S. Srinivasan, Jeremy Wade, and Elias K. Stefanakos

Clean Energy Research Center, College of Engineering, University of South Florida, Tampa, FL 33620, USA

Received 20 January 2006; Revised 29 June 2006; Accepted 13 July 2006

A new coprecipitation/hydrolysis synthesis route is used to create a TiO2-ZnFe2O4 nanocomposite that is directed towards extend-ing the photoresponse of TiO2 from UV to visible wavelengths (> 400 nm). The effect of TiO2’s accelerated anatase-rutile phasetransformation due to the presence of the coupled ZnFe2O4 narrow-bandgap semiconductor is evaluated. The transformation’sdependence on pH, calcinations temperature, particle size, and ZnFe2O4 concentration has been analyzed using XRD, SEM, andUV-visible spectrometry. The requirements for retaining the highly photoactive anatase phase present in a ZnFe2O4 nanocompos-ite are outlined. The visible-light-activated photocatalytic activity of the TiO2-ZnFe2O4 nanocomposites has been compared to anAldrich TiO2 reference catalyst, using a solar-simulated photoreactor for the degradation of phenol.

Copyright © 2006 Sesha S. Srinivasan et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

1. INTRODUCTION

The wide-bandgap semiconductor TiO2 has become thedominant UV-activated photocatalyst in the field of air andwater detoxification because of its high stability, low cost,high oxidation potential, and chemically favorable proper-ties. The demand for visible-light-activated photocatalyticsystems is increasing rapidly; however, currently, the effi-ciency and availability of photocatalysts that can be acti-vated effectively by the solar spectrum and particularly in-door lighting are severely limited. Environmental pollutionon a global scale is proposed to be the greatest problem thatchemical scientists will face in the 21st century, and an in-creasing number of these scientists are looking for new pho-tocatalytic systems for the solution. The vast majorities ofcurrent photocatalytic system use pure or modified TiO2

with a metastable anatase crystal structure (3.2 eV bandgap),although two key shortcomings exist. The first shortcomingis low photocatalytic efficiencies that plague current photo-catalysts due to undesired electron-hole pair (EHP) recom-bination, and the second is that TiO2 utilizes only 3–5%of the solar spectrum and virtually none of the light com-monly used for indoor illumination. Both of these spectralregions have applications needing active photocatalysts [1].The push towards extending the photoresponse of TiO2 tovisible wavelengths is increasing exponentially every year, forboth solar (λ > UV-A, 320 nm) and visible light applications

(> 400 nm). The most successful techniques used thus far forthe development of modified TiO2 for visible-light photocat-alysts are ion implantation methods using Cr or V ions [2],various synthesis techniques [3], and substitutional dopingof nonmetals such as N (TiO2�xNx) [4, 5].

Recent efforts have also been sought to extend the pho-toresponse of TiO2 through charge-transfer interactions withnarrow-bandgap metal oxides such as the n-type ZnFe2O4

with a 1.9 eV bandgap. ZnFe2O4, most prominently knownfor its magnetic properties, is a photocatalyst active for irra-diation wavelengths shorter than 652 nm, although its pho-toactive lifetime is short due to the tendency of absorb-ing intermediate oxidation byproducts, thereby inhibitingit from oxidizing target organics [6]. Numerous publica-tions in the past year have used sol-gel techniques [7, 8]to dope TiO2 with ZnFe2O4 or Zn2+ and Fe3+ ions [9] forsolar-light-irradiated photocatalytic studies. TiO2�ZnFe2O4

alloys, on the other hand, are difficult to prepare becauseof the differences in preparation procedures (ZnFe2O4 istypically prepared through coprecipitation in alkaline solu-tions) and also the enhancement of TiO2’s anatase to ru-tile transformation by the substitutional presence of Fe3+

ions. However, a complex colloidal chemistry method usingsurfactant capping has been reported for creating photoac-tive TiO2�ZnFe2O4 nanocomposites that exhibit increasedphotocatalytic response to solar irradiation [10]. In this re-port, a simple coprecipitation/hydrolysis synthesis method is

2 Journal of Nanomaterials

used to create TiO2�ZnFe2O4 alloys for the purpose of cre-ating an inexpensive and nontoxic photocatalyst that is pho-toactive in response to wavelengths greater than 400 nm.

2. EXPERIMENTAL METHOD

A coprecipitation/hydrolysis synthesis method was used forthe formation of TiO2�(X) ZnFe2O4 nanocomposites withX (mole fraction) values of 0.01, 0.05, 0.1, 0.15, and 0.2.All chemicals used were purchased from Sigma Aldrich andwere of 99.9% purity or higher. ZnFe2O4 was first precipi-tated using the respective quantity of Fe(NO3)3 � 9H2O andZn(NO3)2 � 6H2O precursors in a solution of C3H7OH (iso-propyl alcohol) heated at 65ÆC and stirred for 30 minutes.To coprecipitate the nitrate precursor, the pH of the aqueoussolution was raised to 6.5 by slowly adding a 3.5 M NH4OHsolution using C3H7OH as the solvent. Approximately 10 gof deionized H2O was next added dropwise to the solu-tion in addition to the H2O already existing from the addedNH4OH and the solution was left stirring for 45 minutes.A separately prepared solution of Ti(OBu)4 and C3H7OHwas mixed in a ratio of 1 : 2 by weight and added drop-wise to the coprecipitated ZnFe2O4 solution for a controlledhydrolysis with H2O : Ti(OBu)4 ratios of 50, 25, 15, 5 : 1.The final solution was kept stirring at 65ÆC for 90 min-utes, filtered, dried at 100ÆC and 220ÆC, respectively. Thusprepared TiO2�ZnFe2O4 nonoparticles have been calcinedin a flowing air atmosphere at various temperatures for 3hours.

The UV-Vis and visible-light photoactivity of the nano-composites were determined using photoreactor consistingof a 1000 W (340 < λ < 680 nm) metal halide lamp, reflec-tor, and borosilicate reaction vessel. For visible-light pho-toactivity experiments, a thin-film UV cutoff filter providedby Edmund Optics was inserted on the glass lens of the metalhalide lamp to completely remove irradiation with wave-lengths shorter than 400 nm. Phenol was used as the organictest degradant since it absorbs wavelengths around 265 nmand therefore will not be susceptible to photolysis. Experi-mental reaction conditions for all studies consisted of 40 ppmphenol, 1100 g of deionized H2O, 1 g/L catalyst loading, mag-netic stirring, and 1.5 L/min of air injected through a spargerto perform the role of electron scavenging. The degradationof phenol was evaluated by centrifuging the retrieved sam-ples and measuring the intensity of phenol’s absorption peak(268 nm) relative to its initial intensity (C/Co) by UV-Visspectroscopy with an Ocean Optics fiber optic spectrometer.

3. RESULTS AND DISCUSSION

3.1. X-ray diffraction

To confirm the multiphase synthesis of TiO2�(X) ZnFe2O4

nanocomposites, alloys were formed using a high concentra-tion (X = 0.2) of ZnFe2O4. XRD measurements revealed theformations of anatase and rutile TiO2, as well as ZnFe2O4

and traces of ZnTiO3 and ZnO. Although a low calcination

20 25 30 35 40 45 50 55 60 65h = 15

h = 25

h = 35

h = 50

A

R

� �

�R

RR �R A �

R

� �R

Inte

nsi

ty(a

.u.)

2 theta (degrees)

A = anataseR = rutile

�= spinel ZnFe2O4

� = ZnTiO3

Figure 1: XRD spectra of TiO2�(0.2) ZnFe2O4 calcined at 400ÆCfor various h values.

temperature of 400ÆC was used, the anatase-to-rutile trans-formation was nearly complete due to the inherent substi-tution of Fe3+ for Ti4+ ions, thereby creating oxygen vacan-cies that promote the formation of rutile throughout the bulkof the nanoparticles [11]. The considerable amount of Fe3+

ions needed to realize such a low temperature anatase-rutiletransformation may also explain the formation of ZnTiO3

and ZnO as an incomplete formation of ZnFe2O4 due tothe absence of available Fe. Figure 1 shows the XRD spec-tra of the TiO2�(0.2) ZnFe2O4 nanocomposites using var-ious h values (h = H2O/Ti[OBu]4), since this ratio effectsthe hydrolysis of TiO2 and therefore the photoactivity of thenanocomposite.

Based on the previous experiment, as well as others con-ducted on pure hydrolyzed TiO2 [12], it was decided to useh values of 25 for the remainder of the nanocomposites.This ratio provides the optimal tradeoff between hydroly-sis homogeneity, anatase content, and photocatalytic activ-ity. TiO2�(X) ZnFe2O4 nanocomposites were next synthe-sized using X values of 0.01, 0.05, 0.1, and 0.15 to determinethe maximum concentration of ZnFe2O4 that can be coupledwith TiO2 while still maintaining an anatase crystal structurewhich is predicted to be needed for the redox reactions de-rived from a charge-transfer phenomenon. The XRD spec-tra of the TiO2�X% ZnFe2O4 nanocomposites are shown inFigure 2.

Figure 2 shows the effect of ZnFe2O4 alloying concentra-tion on the anatase-to-rutile transformation. For a calcina-tion temperature of 450ÆC, X values of 0.01, 0.05, 0.1, 0.15,and 0.2 correspond to respective anatase mass fractions of100, 100, 72.7, 54.1, and 47.2%, as calculated by the equation

XA = 11 + 1.26 �

(IA/IR

)� 100, (1)

XA is the mass fraction of anatase, IA and IR are the X-rayintegrated intensities of the (101) reflection of anatase andrutile phases, respectively. It should be noted that this equa-tion only takes into consideration the percent of anatase and

Sesha S. Srinivasan et al. 3

20 25 30 35 40 45 50 55 60 65X = 0.01

X = 0.05

X = 0.1

X = 0.15

X = 0.2

A R� �

�R

A R �R A �AR� A�R

Inte

nsi

ty(a

.u.)

2 theta (degrees)

A = anataseR = rutile

�= spinel ZnFe2O4

� = ZnTiO3

Figure 2: XRD spectra of TiO2�(X) ZnFe2O4 calcined at 450ÆC.

rutile in the formed TiO2 and not of the entire nanocompos-ite.

3.2. UV-visible spectroscopy

An Oriel Instruments spectrometer with an integratingsphere has been used for UV-Vis spectrometry measure-ments used to analyze the redshifts in the absorption re-gions as a function of ZnFe2O4 alloying concentration. Thenanoparticles have been deposited on the glass slides by spin-coating technique at 3000 rpm. The UV-Vis transmittancemeasurements were taken and converted into absorptionreadings. Figure 3 represents the UV-Vis absorption spec-tra for TiO2�(X) ZnFe2O4 nanocomposites with X = 0.01,0.05, 0.1, 0.15, and 0.2. It can be seen that the redshift ofthe absorption edge is roughly proportional to the ZnFe2O4

alloying concentration. The absorption bands were smoothfor low alloying concentrations (X < 0.15), however smallshoulders appeared for higher ZnFe2O4 concentrations as of-ten seen in the absorption bands of doped photocatalysts.

3.3. Photocatalytic studies

The photocatalytic activity of the samples in response toUV-Vis and visible-light irradiation was determined usingphenol degradation over a 105-minute time period, withsamples withdrawn every 15 minutes. The UV photoactiv-ity of the TiO2�(X) ZnFe2O4 nanocomposites was foundto decrease in rough proportion to the increasing concen-tration of ZnFe2O4. The visible-light photoactivity (shownin Figure 4), however, was characterized by a bell curve,with the maximum degradation being achieved with X equalto 0.10. For comparison with the nanocomposite materials,pure ZnFe2O4 prepared by the hydrolysis and coprecipita-tion method has been plotted in this figure. Figure 5 showsa breakdown of the photocatalytic activity of TiO2�(X)ZnFe2O4 nanoparticles from various degradation experi-ments.

350 360 370 380 390 400 410 420 430 440 450 460 470 480 490 5000

0.5

1

1.5

2

2.5

Wavelenght (nm)

Abs

orba

nce

(a.u

.)

UV cutofffilter

TiO2 (Aldrich)

TiO2 (hydrolysis)X = 0.01X = 0.05

X = 0.1

X = 0.15X = 0.2

Figure 3: UV-Vis absorbance spectra of TiO2�(X) ZnFe2O4

nanocomposites.

0 10 20 30 40 50 60 70 80 90 100 110 12075

80

85

90

95

100

105

X = 0X = 0.2X = 0.15ZnFe2O4

X = 0.01X = 0.05X = 0.1

Time (min)

C/C

o(%

)

Figure 4: Visible-light-activated photocatalytic degradation ratesfor TiO2(X) ZnFe2O4 nanocomposites.

The decreasing photoactivity for the catalysts for X val-ues more than 0.1 when irradiated by UV light is attributedto the increasing formation of the rutile phase with ZnFe2O4

concentrations as well as possible defects in the TiO2 lat-tice, in addition to enhanced electron-hole pair (EHP) re-combination. The increased photoactivity for X values of0.01 to 0.10 may be associated with the role ZnFe2O4 substi-tuted or surface-stabilized anatase TiO2 serves in extendingthe photoresponse of the catalyst to short-wavelength visibleirradiation and effectively transferring charge carriers to par-ticles capable of the appropriate redox reactions.

4 Journal of Nanomaterials

X = 0 X = 0.01 X = 0.05 X = 0.1 X = 0.15 X = 0.2

0

10

20

30

40

50

60

VisUVUV-Vis

TiO2�(X)ZnFe2O4

Deg

rada

tion

in10

5m

inu

tes

(%)

Figure 5: Breakdown of the degradation characteristics forTiO2�(X) ZnFe2O4 nanocomposites irradiated by UV, and UV-Vis,and visible light.

The photoactivity that characterizes the nanocompositephotocatalysts using visible-light irradiation is not directly(or at least completely) linked to ZnFe2O4. The reason forthis conclusion is that the consistent degradation rates char-acteristic of the TiO2�(X) ZnFe2O4 nanocomposites are dis-similar to the degradation rates for ZnFe2O4 which werefound in prior experiments to cease after short durations us-ing both UV and visible-light irradiations. It can also be con-cluded from the poor visible-light photocatalytic activity ofnanocomposites with X equal to 0.15 and 0.20 that anataseTiO2 plays a critical role in the visible-light photocatalysis ofTiO2�ZnFe2O4 alloys. Although the ZnFe2O4 concentrationincreases, thereby allowing more charge carriers in responseto enhanced visible-light absorption, it is thought that thehigh rutile concentrations impede the overall reaction.

4. CONCLUSION

TiO2ZnFe2O4 nanocomposites were synthesized, and theirphotocatalytic activity in response to UV, UV-Vis (solar),and visible light was determined. It was concluded that thenarrow-bandgap semiconductor ZnFe2O4 can be effectivelycoupled with TiO2 for visible-light (> 400 nm)-activatedphotocatalysis. This study shows the potential of inexpensiveand nontoxic photocatalysts for indoor visible-light applica-tions.

ACKNOWLEDGMENTS

The authors wish to thank the Nanomaterials and Nanoman-ufacturing Research Center at University of South Florida forextensive use of X-ray diffraction facilities. Financial supportfrom USF, DOE, and Center for Biological Defense are grate-fully acknowledged.

REFERENCES

[1] M. Anpo, “Applications of titanium oxide photocatalysts andunique second-generation TiO2 photocatalysts able to operateunder visible light irradiation for the reduction of environ-mental toxins on a global scale,” Studies in Surface Science andCatalysis, vol. 130 A, pp. 157–166, 2000.

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[7] G.-G. Liu, X.-Z. Zhang, Y.-J. Xu, X.-S. Niu, L.-Q. Zheng, andX.-J. Ding, “Effect of ZnFe2O4 doping on the photocatalyticactivity of TiO2,” Chemosphere, vol. 55, no. 9, pp. 1287–1291,2004.

[8] P. Cheng, W. Li, T. Zhou, Y. Jin, and M. Gu, “Physical and pho-tocatalytic properties of zinc ferrite doped titania under visiblelight irradiation,” Journal of Photochemistry and PhotobiologyA: Chemistry, vol. 168, no. 1-2, pp. 97–101, 2004.

[9] Z.-H. Yuan, J.-H. Jia, and L.-D. Zhang, “Influence of co-doping of Zn(II) + Fe(III) on the photocatalytic activityof TiO2 for phenol degradation,” Materials Chemistry andPhysics, vol. 73, no. 2-3, pp. 323–326, 2002.

[10] Z.-H. Yuan and L.-D. Zhang, “Synthesis, characterizationand photocatalytic activity of ZnFe2O4/TiO2 nanocomposite,”Journal of Materials Chemistry, vol. 11, no. 4, pp. 1265–1268,2001.

[11] F. C. Gennari and D. M. Pasquevich, “Kinetics of the anatase-rutile transformation in TiO2 in the presence of Fe2O4,” Jour-nal of Materials Science, vol. 33, no. 6, pp. 1571–1578, 1998.

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